The term "gauging" is commonly used in the art to designate determining the quantity or content of gas available in the tank, said content being expressed in units of mass, or in certain applications more commonly in liters.
When expressed in liters, the conventional unit is liters NTPD for "Normal Temperature and Pressure, Dry", i.e. the volume the gas would occupy if dry and under standard conditions of temperature and pressure.
In some applications, it is difficult to gauge a high pressure gas contained in tanks accurately because of variations in various parameters that are involved in determining the content of the tank. This problem is made even more complicated in the special case of a supply of oxygen gas on board an aircraft since it is necessary to satisfy safety requirements for which the presence of a high pressure circuit is a cause of particular concern.
It is conventional to fit a high pressure gas tank, in particular a tank of oxygen gas, with a pressure regulator, sometimes also caused an expander, whose function is to regulate the outlet pressure to a stabilized low value, e.g. 0.5 MPa, regardless of the outlet flow rate.
When it is desired to determine the content of the tank, it is common practice to mount a pressure sensor on such a pressure regulator. Such a sensor is powered electrically and delivers a signal proportional to the pressure of the gas in the tank. Sensors used for this purpose include membrane sensors, beam deflection sensors, strain gauge sensors, and piezo-resistive sensors.
Such a pressure sensor is generally associated with a digital readout or with an analog readout using a pointer, with the readout being graduated directly in pressure units, or as a percentage of tank pressure, which corresponds substantially to the percentage to which the tank is full.
This gives an idea of the gas content of the tank, assuming that the other parameters involved in determining said content are constant, which parameters include, in particular, the temperature and the coefficient of compressibility of the gas.
Thus, the measurement performed takes account of gas pressure only, and takes no account of gas temperature T or of its coefficient of compressibility z: as a result the measurement is relatively imprecise.
In the particular case of a supply of oxygen gas on board an aircraft, the variations in these two parameters T and z are large (T varies between 258.degree. K. to 328.degree. K., and z varies between about 1 and 0.9), such that measurement inaccuracy is further increased, and it may be estimated that the relative error can be as much as 25% for a given tank (where the mass m of gas is given by the equation m=PV/zrT, where P is pressure, V is volume, z is the coefficient compressibility, r is a constant for the gas in question, and T is temperature).
The above problem is naturally applicable to the case where a plurality of high pressure gas tanks have their outlets interconnected, as is the case in particular for oxygen tanks on board aircraft.
Two types of circuit are conventionally used with a plurality of tanks.
One conventional system design has a common high pressure duct to which the outlets of all of the tanks are connected, the duct leading firstly to a pressure regulator having a low pressure utilization circuit connected downstream therefrom, and secondly to a single sensor which delivers a signal proportional to the pressure in the common duct.
This thus serves to measure pressure and the corresponding signal is transmitted to a readout graduated in terms of pressure or in terms of percentage full, or even in terms of liters NTPD content, as mentioned above.
In some cases, the sensor-readout assembly is replaced by a mere pressure gauge which, while simplifying the structure, nevertheless further increases the inaccuracy of the measurement performed.
Such a design suffers from numerous drawbacks.
Firstly, when the tanks contain oxygen gas, the presence of a circuit containing oxygen gas at high pressure represents a danger due to the risk of spontaneous ignition (in particular because of particles of grease and/or metal that may escape prior cleaning of the circuit), together with the dangers inherent to having high pressure connections (dangers of breakage, snagging, or even of a tube being pulled off).
This danger is particularly serious for a supply of oxygen gas on board an aircraft, since in addition to the above-mentioned risks, including a fire in the hold of the aircraft where the tanks are located, such a failure can empty the tanks and reduce the available supply of oxygen gas to nil (the tanks are naturally not designed to be refilled in flight).
In addition, the measurements are highly inaccurate, as explained above, since they take account only of the pressure of the gas in the tanks. For a supply on board an aircraft, the pilot has difficulty in establishing the real content oxygen available in the gas tanks before take-off, and this means that it is necessary to provide a very wide safety margin.
In practice, if more accurate determination of the content in the tanks is desired, then tables are used to correct the measurements performed by taking account of the coefficient of compressibility and of the temperature of the gas (when known): nevertheless, this procedure is onerous and error-prone.
Attempts have been made to remedy some of the above drawbacks by modifying the design of the system, in particular to avoid the presence of a high pressure circuit.
The modified design consists in fitting each tank with its own pressure regulator, together with a pressure sensor and an associated readout.
The advantage then lies in the connections with the common duct being made downstream from the pressure regulators so that the utilization circuit no longer includes a high pressure portion.
However, processing is increased and reading made more complicated. It becomes necessary to sum the contents of the various tanks in order to obtain the total content, which constitutes a more complex operation, and which further degrades accuracy since the individual content measurements already contain a high degree of error (in other words, this system merely reproduces the above-described system, but on a per tank basis). Further, bulk is necessarily increased and this constitutes a significant disadvantage in relation to a supply of oxygen gas on board an aircraft.
An object of the invention is to provide gauging apparatus that avoids the above-mentioned drawbacks, in particular enabling the high pressure network to be limited or even eliminated and also making it possible to determine the overall content available in the tanks both accurately and quickly.
Another object of the invention is to provide gauging apparatus making it easy to provide a direct indication of overall available capacity in liters NTPD, and also making it possible to display other data, such as the contents available in each individual tank, without the need to refer to correction tables.